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What are blood types?

Dr Karen Martin
Reviewed by Dr Karen MartinReviewed on 19.10.2023 | 3 minutes read

There are four main blood groups, determined by a system called the ABO system. Your blood group is inherited from the genes of both of your parents.

To understand your blood type, you must first understand that each type comprises antibodies and antigens. Antibodies are plasma proteins that help your immune system to defend against foreign substances like germs. Antigens are a different type of protein that is found on the surface of a red blood cell. Think of antigens like a jacket, and your body can recognize that jacket as either your own or as someone else’s and, therefore, a threat that needs to be fought off.

If you are blood group A, you have A antigens on the surface, with anti-B antibodies in the plasma. Those with blood group B have B antigens on the surface with anti-A antibodies. If you are in blood group O, you have no antigens but antibodies against both groups A and B. Those with blood group AB have A and B antigens and antibodies against no blood groups.

Most people in the US are in blood group O.

Why is it important to know your blood group?

There are times when people need a blood transfusion, such as if they’re losing blood in an operation or from an injury or accident. This transfusion comes from a bank of blood that others have donated. It’s been tested and vetted as safe and categorized into each blood group from the ABO system.

You need to have a transfusion compatible with your blood group. Otherwise, your immune system will mount a response and attack it, which can be life-threatening. Blood group A can accept blood group A blood, but not group B, as this will be seen as foreign by the anti-B antibodies. The same goes for if you are blood group B, you cannot receive A blood.

If you have group AB, you are a universal acceptor, as you can receive any blood type as a transfusion.

On the other hand, no one has antibodies to blood group O, so this is the universal donor – it can be safely given to everyone. This is important if it’s an emergency and the lab hasn’t yet processed which blood group a patient is in.

Those with blood group O can only receive blood group O.

What is Rhesus blood type?

In addition to the four blood groups, the red blood cells have another antigen called the Rhesus D antigen. Individuals are Rhesus D positive (RhD +ve) if it is present. They are Rhesus D negative (RhD -ve) if it is not present.

Most people in the US are Rhesus D positive.

What is the importance of the Rhesus blood type?

Knowing your blood type is important if you are required to get a blood transfusion. In an emergency where you lose a lot of blood and need this replaced immediately, there may not be time to check your blood group. Therefore the safest blood group to give to others is O negative, that's also Rhesus D negative, as there are no antigens on the surface for the recipient to react to.

It’s always important that pregnant women know their blood type. This is because the mixing of blood types between mother and child can cause a reaction from the mother if the mother is Rhesus D negative and her growing baby is Rhesus D positive.

She will build antibodies that may not affect her first pregnancy but may mount a response to future pregnancies and cause a life-threatening condition called Rhesus disease in the newborn. Rhesus D negative mothers will receive immune suppression to prevent this response.

How do I find out my blood type?

Your doctor doesn’t routinely check your blood group. It will be checked in an emergency, before invasive surgery, if you are pregnant, and if you are going to give blood.

Blood groups have sometimes been used in custody battles if a child has a different blood group than their father. If you are involved in this situation, it’s important to gather all the facts before drawing conclusions. Genetics is complex, and an expert can take you through the chances of a child having a particular blood group based on their mother’s and father’s blood groups. It doesn’t necessarily follow that children have the same blood group as either parent.

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This article has been written by UK-based doctors and pharmacists, so some advice may not apply to US users and some suggested treatments may not be available. For more information, please see our T&Cs.
Dr Karen Martin
Reviewed by Dr Karen Martin
Reviewed on 19.10.2023
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